Photo detectors such as optical sensors, light-emitting devices such as LEDs, and optical signal transmission devices such as optical switches are used as light-based devices (so-called optical devices), in every field of industries. Among these, applications of solid-state imaging devices such as CCDs and CMOS image sensors manufactured by applying a semiconductor microfabrication technique have been increasingly enhanced. Furthermore, development and application of optical MEMS devices having functions of these optical devices incorporated in a micro machine element are rapidly underway. To optical MEMS devices of this type, not only a Si substrate for which a microfabrication technique has been established but also materials having high light transmission, particularly SiO2 materials (such as synthetic quartz and glass) are often applied.
In manufacturing a solid-state imaging device or an optical MEMS device of this type, a wafer level process is generally used in which a plurality of devices are collectively formed on a semiconductor wafer and sealed, and the semiconductor wafer is divided into individual devices by dicing.
The wafer level packaging is a technique which involves layering and bonding a sealing wafer onto a device wafer on which the devices are formed, to seal the devices in this manufacturing process. In recent years, a manufacture method using direct bonding for bonding the wafers is proposed. Since no other material layers are present on the interface between the wafers in the bonding based on the direct bonding without using adhesive or solder, it is advantageously possible to ensure high bonding strength and favorable interface physical properties. Examples of direct bonding conventionally used to manufacture an MEMS device include anodic bonding and diffusion bonding. Furthermore, there is recently proposed a bonding method for obtaining strong bonding with hydrogen bondings and heat treatment through preparing hydroxy groups or the like on a flattened and cleaned surface.
These direct bonding methods are, however, accompanied by heat treatment in the bonding step or after bonding, and this causes a problem in the manufacture of the MEMS devices. For the optical MEMS device, optical transparency is required for packaging, and therefore sealing is desired which uses an optically transparent substrate (such as SiO2 base material), which is different from the device substrate (such as Si). When the bonding step of such different substrates are accompanied by heat treatment, a heat strain is generated on the bonding interface due to the difference in the thermal expansion coefficient between the substrate materials, which deteriorates reliability and durability of the devices. Therefore, reduction of the bonding process temperature is a major technical problem with the manufacture of MEMS devices. It is another manufacture problem that the tact time of the manufacture cannot be shortened since it takes time for heating and cooling.
Therefore, it is desired to apply a room-temperature bonding process that is not accompanied by heating in the bonding step; however, the bonding performance largely depends on the physical properties of the material to be bonded in the room-temperature bonding, particularly that achieving bonding with dangling bonds of the surfaces to be bonded without applying active groups to the surfaces. Especially, it is known to be difficult to bond the SiO2 base material to a target substrate by the room-temperature bonding.
While the room-temperature bonding has been known as a metal bonding method, the room-temperature bonding is has been gradually applied to bonding of semiconductor material or oxide material. However, as described in Takagi et al., Proceedings of NEDO (New Energy and Industrial Technology Development Organization), 2003 Research Promotion Business Accomplishment Report Meeting, pp. 220-225 (2003), it is known that considerable bonding strength can be obtained for some of oxide materials such as Al2O3 through surface activation and pressure application, while practical bonding strengths cannot be obtained for materials such SiO2. Accordingly, there is proposed a method using a surface treatment and a treatment after heating such as application of active groups onto the surfaces to be bonded.
Japanese Laid Open Patent Application No. P2004-054170A discloses a method of bonding laser optical crystals. This method is characterized by bonding the laser optical crystals by performing only ion beam etching on the bonding surfaces without using any intermediate material such as adhesive. This method is a technique developed as a method of bonding laser optical crystals, particularly YVO4 crystals. Although this method is applicable to bonding of a garnet crystal, this method cannot bond SiO2 base material.
Japanese Laid Open Patent Application No. P2005-104810A discloses a method of performing room-temperature bonding for a functional ceramics polycrystal to a semiconductor single crystal such as Si. This method is characterized by involving forming a metal thin layer having reaction activity to semiconductor on the surface of the ceramics polycrystal, and bonding the ceramics polycrystal to the semiconductor single crystal with the reaction product generated by a reaction of metal to semiconductor. This method is proposed as an effective technique for bonding a ceramics substrate having a high surface roughness.
However, these methods suffers from a problem that a heating treatment is required as a step after the bonding when the methods are applied to the MEMS device manufacture. Furthermore, target materials are limited because of the premise of reactivity between the bonding target substrate and the metal layer.
On the other hand, Takagi, Mechanical Engineering Laboratory Report 189 (2000) or the like has pointed out that even a hard-to-bond material such as SiO2, for which strength cannot be obtained only with surface activation and pressure application, can be room-temperature-bonded by forming a metal film on the surfaces to be bonded. Methods of specifically executing this have been proposed so far.
Japanese Laid Open Patent Application No. P2004-337927A discloses a technique characterized by forming a metal thin film on a to-be-bonded surface as a method of bonding ionic crystal substrates that are hard to bond by room-temperature bonding. With this technique, an inert gas ion beam or inert gas neutral atom beam and a metal ion beam or metal neutral atom beam are irradiated onto the to-be-bonded surfaces in vacuum to form a metal thin film having a thickness of 1 nm to 100 nm on the to-be-bonded surface of each substrate.
Japanese Laid Open Patent Application No. P2004-343359A discloses a method of manufacturing an elastic surface wave device by room-temperature bonding and refers to a bonding method with an intermediate film as a method therefor. The method is characterized by bonding a piezoelectric single crystal substrate made of LiTaO3 or the like to a crystalline substrate made of Al2O3, Si or the like by a surface activation treatment and pressure application without a heat treatment at high temperature. As one example of this method, a method of bonding the both substrates by forming Si or insulating material or a metal layer as the intermediate layer.